creating a pivot that documents monthly differences

[Firms]

Hi,

I am rather new to Qlikview,  and I am trying to document a monthly changes to a subtotal via a pivot.

For Example

I really want to achieve a second row documenting the difference between the monthly columns. So, under February, I would see -7,518 and under March; -2,843 etc... Obviously January will be NULL (but I can live with that).

The only way I can think about doing this is in a separate pivot, but I wouldn't even have a clue where to start with an expression to make this work.

Any thoughts would be greatly appreciated.